Vehicle mounted cranes and digger derricks are commonly used for a variety of construction and load handling jobs. For example, a digger derrick is useful in setting utility poles in the ground. The auger of the digger derrick is operated to dig a hole in the ground, and the machine is then used to raise the pole and set it in the hole. Various objects can be lifted to elevated positions by raising the boom with the boom lift cylinder or the winch.
Digger derricks and cranes of this type include a pedestal which is mounted in the bed of the vehicle. A rotation bearing supports a turntable on top of the pedestal, and the boom is mounted on the turntable so that it can be pivoted up and down about a horizontal boom pin. A hydraulic lift cylinder is connected between the turntable and boom to raise and lower the boom. Typically, the boom is a multiple stage unit having two or more sections that extend and retract telescopically. In the case of a digger-derrick, the boom may be provided with a variety of equipment, including a hydraulically driven auger for digging operations, a pole guide for use while raising and setting utility poles, a winch and sheave for raising and lowering loads, a jib for material handling operations, a platform for allowing personnel to work at elevated positions near the boom tip, and various other accessories. The vehicle is normally equipped with outriggers which provide stability and resist tipping of the vehicle when the boom is extended to the side and subjected to heavy loading.
In the design of equipment of this type, the lifting moment applied to the boom by the hydraulic lift cylinder is an important parameter. The lifting moment is equal to the force applied by the cylinder multiplied by the distance between a line coincident with the cylinder center line and another line which passes through the boom pivot pin and is parallel to the cylinder center line. For a conventional lift cylinder, the force component is equal to the area of the piston times the available hydraulic pressure (adjusted by friction losses, back pressures and the like). A pressure control device of some kind normally sets the maximum allowable hydraulic pressure, and the area of the piston is constant. Therefore, the maximum force component of the lifting moment is fixed. The distance component of the lifting moment (moment arm) in a conventional machine depends upon the locations of the cylinder attachment points to the turntable and boom relative to one another and to the boom pivot pin. Because all three locations are fixed, the distance component of the lifting moment is fixed at any given boom angle, although it varies as the boom angle changes. These factors are explained in more detail in SAE Technical Paper No. 830194 entitled "Standardization of Digger Derrick Capacity Ratings" by Frank D. Freudenthal (1983).
Because the maximum force component of the lifting moment is constant, the maximum lifting moment occurs at a boom angle where the distance component of the moment is maximum. As the boom is raised or lowered from this position, the moment decreases because the distance component of the lifting moment decreases. The maximum lifting moment is one of the principal factors that controls the structural design of the machine because the strength of all structural components (such as the chassis frame, the boom assembly and the turntable), as well as the outrigger design and the size and rating of the main rotation bearing, must be sufficient to withstand the maximum lifting moment. Consequently, even though the maximum lifting moment is encountered at only one position of the boom, it dictates the structural requirements that must be met. The result is that the machine structure is not fully utilized during the vast majority of its operating conditions when the boom is oriented at a position other than the maximum moment position.